p-Vinylphenol-based polymers are in use in various industrial fields, as functionalized polymer materials such as photosensitive resin, thermosetting resin, coating material, material for rust preventive, and the like. When an ordinary unpurified p-vinylphenol monomer is used as a polymerization raw material, however, the obtained polymer (p-vinylphenol-based polymer) has a yellowish brown color and has had limited applications.
Further, in recent years, p-vinylphenol-based polymers have drawn attention as a substance essential for use in a photoresist for production of super integrated semiconductor circuit, in the field of photosensitive electronic materials. In order to use a p-vinylphenol-based polymer in this photoresist application, the solution of the polymer is required to be superior in transmittances of visible light and far-ultraviolet light (KrF excimer laser beam of 248 nm) as well as in heat resistance. The heat resistance of a p-vinylphenol-based polymer can be evaluated by, for example, glass transition temperature; in order for the polymer to have excellent heat resistance, the polymer is required to have a molecular weight not lower than a certain high value, specifically a weight-average molecular weight of about 7,000 or higher. Incidentally, this specific value of molecular weight differs depending upon the kind of the polymer.
As to the process for production of p-vinylphenol-based polymer, various processes are known and they can be largely divided into two kinds of processes.
The first process comprises subjecting a p-vinylphenol monomer which is obtained by hydrolysis of acetoxystyrene, decarboxylation of hydroxycinnamic acid, catalytic decomposition of bisphenolethane, dehydrogenation of ethylphenol or the like, to homopolymerization or copolymerization in the presence of a radical polymerization initiator or a cationic polymerization catalyst. The second process comprises subjecting a monomer which is obtained by protecting the hydroxyl group of p-vinylphenol with an acetyl group, a trialkylsilyl group, a tert-butyl group, a tert-butoxycarbonyl group or the like, to homopolymerization or copolymerization in the presence of a radical polymerization initiator or an anionic polymerization catalyst and then removing the protective group for reversion to hydroxyl group.
Since a p-vinylphenol monomer is used as a polymerization raw material, the first process has a merit of being able to obtain an intended p-vinylphenol polymer directly. However, the p-vinylphenol-containing reaction mixture obtained in the synthesis of p-vinylphenol monomer contains impurities considered to hinder the formation of polymer, as part of coloring-causing substances; therefore, in the cationic polymerization of p-vinylphenol monomer, it becomes necessary to purify the p-vinylphenol monomer which is a polymerization material. However, since p-vinylphenol is very unstable thermally and causes polymerization easily, it is difficult to increase its purity to a high level by distillation. Hence, for example, JP-A-51-39788 discloses a method of repeating recrystallization using a hydrocarbon type solvent such as hexane or the like. This method enables production of a polymer of high molecular weight and substantially no coloring, but has not been fully satisfactory for practical application because the purification method by recrystallization is complex and brings about large loss of intended product.
As to the radical polymerization of p-vinylphenol monomer, for example, JP-A-61-291606 or JP-A-63130604 discloses a method of polymerizing a p-vinylphenol monomer in the presence of a phenol having no unsaturated side chain and water for suppression of runaway reaction. In this method, however, since the p-vinylphenol-containing reaction mixture obtained in the synthesis of p-vinylphenol monomer contains coloring-causing substances as mentioned previously, the polymer obtained from the reaction mixture per se (not subjected to any purification) is colored and has been insufficient in transmittances of visible light and far-ultraviolet light. Further, since the concentration of the raw material p-vinylphenol monomer in the reaction system is inevitably low, it has been difficult to obtain a polymer of high molecular weight.
Meanwhile, in the second process, the hydroxyl group-protected p-vinylphenol, unlike the above-mentioned p-vinylphenol monomer, is generally distillable. Therefore, the second process has a merit of being able to polymerize a high-purity monomer obtained by distillation. In the second process, however, it is necessary to remove the protective group of a formed polymer from the polymer in order to obtain an intended polymer; therefore, coloring of polymer may occur in the removal of the protective group and the post-treatment of the formed polymer increases the number of process steps, which have been the drawbacks of the second process.
The object of the present invention is to solve the above-mentioned problems of conventional processes for production of p-vinylphenol polymer and provide a process for producing a light-colored polymer superior in transmittances of visible light and far-ultraviolet light and high in molecular weight, efficiently and economically. As a target for improvement in coloring in visible light wavelength region, a transmittance of 90% can be employed because coloring is hardly noticeable at a light transmittance of higher than 90% and is easily noticeable at a light transmittance of lower than 90%.